Expanded polystyrene is used in large quantities as a packaging material of various products and as a thermal insulation material for the building industry and refrigerators, as a result of its characteristics of low thermal conductivity and good shock absorbance capacity. The waste deriving from these materials and the production scraps of expanded polystyrene are extremely bulky, as their specific weight is very low, and consequently their transportation and disposal in dumps create various problems. The disposal of these waste materials by incineration is also a problem, as it can interfere in the combustion process of the incineration oven and can also produce toxic gases. Some types of expanded polystyrene containing brominated flame-retardant additives, when incinerated, can in fact produce polybrominated dioxins, which are extremely toxic. For these reasons, the waste material from expanded polystyrene should be recycled, after a first step of volume reduction, effected in the production or waste collection sites, followed by the regeneration of the polystyrene, carried out in a centralized recovery plant.
The traditional methods for reducing the volume and the recycling of polystyrene, which envisage thermal treatment, do not allow the complete separation of polystyrene from the other products present in expanded polystyrene, and also have the great disadvantage of causing the partial oxidative degradation of the polymer, thus reducing its quality (Kano, Suzuki, J. Jpn. Pack. Inst., 31, 33, 1993; Sasao, Harade et A1., Kagaku Kogyo, 66, 395, 1992).
Another volume reduction method of expanded polystyrene described in the known art, which overcomes the above described drawbacks, is to dissolve the polymer in an organic solvent. Organic solvents which can appropriately dissolve polystyrene are aromatic hydrocarbons such as toluene and xylenes (U.S. Pat. No. 4,031,039) and halogenated solvents, such as methylene chloride, perchloro ethylene (U.S. Pat. No. 5,891,403). These solvents have the disadvantage of being flammable or dangerous for human beings and the environment. The use of solvents such as d-limonene has been proposed for overcoming these drawbacks (Noguchi, Miyashita, et al., Packag. Technol. Sci. 11, 19-27 (1998), which however has a strong smell of lemon and a low flash point (47° C.); glycol alkyl ethers, some of which are harmful or even toxic, such as diethylene glycol dimethyl ether; dialkyl esters of organic acids, such as dimethyl adipate, dimethyl glutarate and dimethyl succinate (U.S. Pat. No. 5,629,352). When the recovery of polystyrene from the solution is carried out by the evaporation of the solvent, the use of high boiling solvents, such as d-limonene, glycol dialkyl ethers and dialkyl esters of organic acids has the disadvantage of requiring, during the distillation phase of the solvent, very low pressures and the use of costly distillation equipment, in order to avoid the decomposition of polystyrene. In order to solve this problem, the recovery of polystyrene from the solution is effected, in U.S. Pat. No. 5,629,352, by means of precipitation by the addition of water. These methods, as also those which envisage the recovery of polystyrene by evaporation of the organic solvent, have the disadvantage of not allowing the separation from polystyrene of the majority of additives present in the expanded polystyrene, such as, for example, brominated flame-retardant additives, which are soluble in said organic solvents and insoluble in water. In the patent WO 0238659, the volume of the expanded polystyrene is reduced by treating it with a series of solvents consisting of a solvent capable of dissolving polystyrene, such as, for example, dialkyl esters of organic acids, and a non-solvent of the polymer, such as, for example, ethylene or propylene glycol and others. This treatment does not dissolve the expanded polystyrene but simply collapses it. The collapsed polystyrene, in gel form, is then separated and dried. Other methods similar to the previous one, which differ in the series of solvents and in the equipment used, are disclosed in patents WO 0222337, U.S. 2002/0120020. With these methods, the polystyrene, separated in gel form, withholds impurities and large amounts of solvent, which are difficult to remove during the drying phase.
The drawback of all these methods is that it is impossible to obtain pure polystyrene, without all the foreign components normally present in expanded polystyrene, such as labels, other polymers, additives and other materials. In patents EP 0894818 and WO 0214413, the recycling of the polymers in general with separation from foreign materials and additives is obtained by dissolving the polymeric material in a suitable solvent, by separating the insoluble components and recovering the desired polymer by precipitation with a non-solvent. These patents do not provide examples of the recycling of expanded polystyrene and the solvents used for dissolving the polymeric material are, for the most part, flammable, toxic or harmful.
It has now been found that the drawbacks of the known art can be overcome if a dialkyl carbonate is used as solvent for dissolving expanded polystyrene.